Contact lenses are generally classified into hydrophilic contact lenses and hydrophobic contact lenses. Irrespective of the types of contact lenses, soils such as proteins, lipids or inorganic substances derived from tear or sebum will deposit on such contact lenses, and it is necessary to clean them periodically. Especially, in the case of hydrophilic contact lenses, bacteria or fungi are likely to deposit thereon, and it is also necessary to disinfect them by a method such as thermal or chemical disinfection.
To remove protein soils deposited on a contact lens, it has been common to clean the contact lens with a cleaning agent containing a protease. However, proteases are instable in an aqueous solution. For example, a commercially available serine protease derived from bacteria of genus Bacillus (Subtilisin A, manufactured by Novo Nordisk Bioindustry Japan K.K.) loses its enzymatic activity by about 70%, when it is kept in an aqueous solution at pH 7 at 25.degree. C. for 24 hours. Therefore, when a cleaning agent containing a protease is to be provided in a liquid form, it is necessary to incorporate an enzyme stabilizer to maintain the enzymatic activity for a long period of time.
For example, Japanese Unexamined Patent Publications No. 180515/1989 and No. 168224/1990 propose to incorporate a water-miscible organic liquid in a proportion of at least 50% by weight for stabilization of an enzyme. Further, Japanese Unexamined Patent Publications No. 93919/1992 and No. 143718/1992 propose to incorporate a polyhydric alcohol in a proportion of at least 20% by weight. Still further, Japanese Unexamined Patent Publication No. 161920/1992 proposes a cleaning agent containing a saccharide as an enzyme stabilizer.
However, each of these cleaning agents contains an enzyme stabilizer at a high concentration, so that its osmotic pressure exceeds a physiological level, whereby there will be a problem of deforming contact lenses or irritating eyes, when it is applied to hydrophilic contact lenses as it is. Accordingly, when such a cleaning agent is used for cleaning or preserving hydrophilic contact lenses, it is necessary to dilute it with a large amount of purified water or a preserving solution having a low osmotic pressure, to lower the osmotic pressure to a physiological level.
However, such a method wherein a protease-containing cleaning agent must be diluted every time for each cleaning treatment of contact lenses, not only requires a cumbersome diluting operation, but also has a problem that if the cleaning agent is not diluted homogeneously, a deformation is likely to result in the contact lenses thereby treated, or that if the cleaning agent is not diluted to a proper concentration, the osmotic pressure will be improper, which brings about irritation to eyes. Further, if such a cleaning agent is diluted to a physiological osmotic pressure level, the enzyme stability will deteriorate extremely in a short period of time, and the cleaning effect will thereby be lowered, whereby it is impossible to clean hydrophilic contact lenses during their preservation.
Further, Japanese Unexamined Patent Publication No. 121416/1985 proposes a method wherein a contact lens is heated in an aqueous solution containing a protease to simultaneously clean and disinfect it. However, the method also has a problem that the protease is easily inactivated in an aqueous solution, and stabilization of the protease is desired.
Though boric acid and borax are often used in an enzyme-containing cleaning agent as buffer agents (Japanese Unexamined Patent Publication No. 121417/1985), they are also known to have an enzyme stabilizing effect, when used in combination with a polyhydric alcohol (Japanese Unexamined Patent Publications No. 51015/1992 and No. 143718/1992). However, this combination cannot practically be applied to a cleaning agent for hydrophilic contact lenses, because the osmotic pressure will thereby be above the physiological level.
On the other hand, an attempt has been made to improve the cleaning effect of an enzyme-containing cleaning agent. On the basis that inorganic salts, such as calcium salts, in tear are likely to form complexes with proteins, which deposit on surface of contact lenses, it is known to incorporate a metal chelating agent to such a cleaning agent to remove such soils and thereby to improve the protein-removing effect by the enzyme.
However, a metal chelating agent is known to lower the stability of a serine protease extremely. Therefore, it has been common to add a metal chelating agent, not to a protease-containing cleaning agent, but to a diluting solution for such a cleaning agent, so that the metal chelating agent is separated from the enzyme until the cleaning agent is used for treatment. Therefore, users have been obliged to have both an enzyme-containing cleaning agent and a diluting solution.
Further, it has been proposed to incorporate a metal chelating agent and a protease into the same solution, by using a certain specific metal chelating agent to minimize the adverse effect to the enzyme stability (Japanese Unexamined Patent Publication No. 51015/1992). However, this method has a disadvantage that the metal chelating effect is thereby weak, so that the cleaning power will be low.